Patent GB-775797-A (GB)
Title: Apparatus including electrical transmission networks, particularly for the artificial production of speech
IPC: G10L19/00
Filed: 1955-05-06
Abstract: 775,797. Secret transmission. WESTERN ELECTRIC CO., Inc. May 6, 1955 [May 7, 1954], No. 13192/55. Class 40 (4). [Also in Group XL (b)] Apparatus for the artificial production of voice sounds comprises a number of tunable L.C.R. circuits, e.g. 22, 24, 26, Fig. 1, arranged in parallel between a source and a load, a phase-inverting device being interposed in series with each alternate circuit between the source and the load. Each L.C.R. circuit is tunable to a formant frequency of a voice in such a way that the whole network simulates the impedance function of the vocal tract. As applied to a communicating system the signals controlling the circuits 22, 24, 26 are supplied over lines 20 from an analysing network whose input is derived from a microphone 10. Speech frequencies are delivered in parallel to three band-pass filters 12, 14, 16 within whose bands lie respectively the first, second and third frequency formants of the voice. The outputs of the filters are applied to frequency measuring devices 12a, 14a, 16a which deliver formant frequency control signals, and to amplitude measuring devices, e.g. rectifiers 12b, 14b, 16b which deliver formant amplitude control signals. Each control signal passes through a low-pass filter 12c, 12d, &c., which restricts the band it occupies to 25 c/s. In addition, the speech energy is applied to a pitch detector 18 which delivers a control signal representative of the fundamental frequency of the voice. The detector 18 may be as described in U.S.A. Specification 2,522,539. The pitch control signal controls at the receiving end a device 28 which comprises a buzz source and a hiss source, the buzz source being operative during the transmission of voiced sounds and the hiss source replacing the buzz source during the transmission of unvoiced sound when the control signal falls below a certain amplitude level. The detecting device may be as described in U.S.A. Specification 2,522,539. Each tuned synthesizing circuit 22, 24, 26 may comprise the parallel L.C.R. circuit of Fig. 3 which is fed from the buzz/hiss source 28 through a pentode 58 in such a manner that the current supplied is independent of frequency and the voltage derived across the L.C.R. circuit is proportiona to its impedance at each frequency. It can be shown that the magnitude of the impedance of the L.C.R. circuit at resonance and the width of the resonance curve are independent of L so that by changing L the resonance of the circuit may be shifted on the frequency scale without altering the characteristic transfer impedance function of the circuit. To this end, the inductance L comprises a pair of windings on saturable cores 52 which also carry a control winding 54 to which the formant frequency control signal is applied through a pentode 56. Since the inductance L is constituted by a saturable reactor, the resonant frequency of the L.C.R. circuit as determined by the controlled value of L will be proportional to the amplitude of the frequency control current. An auxiliary input 2 from the buzz/hiss source 28 actuates a switch 68 which connects an extra resistance Ru in shunt with Rv when a hiss is being supplied. This increases the damping of the circuit to an appropriate level for unvoiced sounds. The amplitudes of the output voltages from the tunable circuits 22, 24, 26 are determined by the variable gain amplifiers 32, 34, 36 whose gains are controlled by the amplitude control signals. In order to obtain an overall transfer impedance function which does not fall to zero between successive resonance peaks (e.g. the function shown in Fig. 2) it is necessary to insert a phase inverter in the output of every alternate synthesizing filter, e.g. in the embodiment shown a single phase inverter 42 is inserted between the output of filter 24 and the adding network 38. This phase inversion is necessary because of the 180 degree phase difference between impedances at frequencies on the upper shoulder of one resonance peak and the lower shoulder of the next highest peak. The synthesizing circuit may duplicate a transfer junction having more than three peaks and the self-explanatory Fig. 4 shows a circuit which incorporates six resonant L.C.R. circuits TC1, TC2, &c. and provides six peaks. The arrangement of two adders 70, 72 and one subtractor 74 introduces the necessary phase inversions into the circuit. The parallel resonant L.C.R. circuits may be replaced by series resonant circuits fed from low-impedance sources such as cathode followers (Figs. 5, 6, not shown). Tuning of such circuits is accomplished by varying the capacitance element which may comprise a reactance tube having a control terminal to which a frequency-control voltage is applied. The Specification gives values for elements in a typical synthesizing circuit and includes a short theoretical discussion of the invention. Specifications 466,327 and 547,505 also are referred to.